In known bearing assemblies for machine tools, the bearings are subjected to dynamic axial and radial loading forces. It is generally desirable to provide variable loading forces, hereinafter called "preloading", to control stiffness of the bearings in machine tool spindles, such as lathe and milling machine spindles, adapted to hold cutting tools or work pieces.
More specifically, axial loading of bearings (such as an anti-friction ball bearing of an angular-contact variety or a tapered roller bearing) consists of axially biasing one race, such as the outer race, relative to the inner race. When this is done, e.g., in a ball bearing, a force is transmitted from that outer race to the balls and from the balls to the inner race. These forces tend to hold the balls tightly between the races. The greater the axial loading, the more tightly the balls are held. The outer race is tightly secured in a bushing or housing and the inner race is tightly secured on the spindle. Thus, the more tightly the balls are held between races, the more rigidly the spindle is mounted in the housing. At the same time, however, the more tightly the balls are held between races, the more likely it is that the useful life of the bearings may be decreased if the preload forces are excessive.
Cutting loads frequently vary rapidly during maching operations, causing torque load and deflection forces acting on the spindle to vary accordingly. Any looseness in such an environment will cause a loss of accuracy due to excessive ranges of cutter deflection, machine vibration and spindle chatter. Preloading minimizes shake and chatter and reduced deflection during use of the spindles by causing them to be more rigidly held. Preloading also has certain adverse effects. Friction forces in preloaded spindles can increase with preload, and in a spindle which must be operated at high speed, excessive heat normally is generated unless the preload is low in magnitude. Moreover, high operating temperatures and internal forces also may create unpredictable thermal loading detrimental to machine accuracy and bearing life.